1. Field of Invention
The present invention relates to a mask inspection method. More particularly, the present invention relates to a chrome-less mask inspection method.
2. Description of Related Art
In semiconductor fabrication, patterns are transferred through a set of masks. In other words, quality of the mask directly affects the pattern resulting from the transfer. Therefore, a newly fabricated mask must be inspected to ensure the mask is defect free before actual use.
In general, there are two mask inspection methods. One method is called the die-to-die inspection method. In the die-to-die inspection method, the patterns in neighboring units are compared to detect any discrepancies. Another method is called the die-to-database inspection method. In the die-to-database inspection method, the actual pattern is compared with the designed pattern. For the die-to-die inspection method, if the defects on two neighboring units are at the same location, the defects may not be found. On the other hand, since the design pattern is the basis for producing the mask, the design pattern is often verified with great care. Hence, the die-to-database inspection method has a much higher accuracy and is particularly suitable for conducting an initial mask inspection.
However, the die-to-database inspection method is unsuitable for inspecting a chrome-less mask. FIGS. 1A to 1C are a series of diagrams including a top view and sectional view of a conventional mask with a light-blocking pattern and a chrome-less phase shift pattern thereon and a signal trace after traversing the mask. In fact, FIG. 1A is a top view of a conventional mask with light-blocking pattern and chrome-less phase shift pattern thereon. FIG. 1B is a cross-sectional view along line 1B—1B of FIG. 1A. FIG. 1C is an inspection signal pattern after inspecting the mask by scanning along line 1B—1B. As shown in FIGS. 1A and 1B, the mask 100 includes, for example, a transparent region 110, a phase shift region 120 and a light-blocking region 130 on a transparent substrate 102. The transparent region 110 and the phase shift region 120 are formed, for example, by cutting up the transparent substrate 102 into separate blocks each with a different height. The light-blocking region 130 is formed, for example, by depositing chromium material on specific area of the transparent substrate 102 to form a chromium layer 104. As shown in FIG. 1C, a signal trace 150 of this chrome-less mask 100 has a corresponding down-spike 152 at the phase shift region 110 and the transparent region 120.
FIG. 2A shows a first mask database (the design pattern) 160 inside a built-in database for designing and fabricating the chrome-less mask 100. The first mask database 160 includes a phase shift region corresponding to the phase shift pattern 162 and a light-blocking region corresponding to the light-blocking pattern 164 for patterning out the transparent region 110, the phase shift region 120 and the light-blocking region 130 of the chrome-less mask 100. Using the first mask database 160, a mask with a cross-sectional structure as shown in FIG. 2B is produced. The mask has a chromium layer 104 over both the phase shift region 110 and the light-blocking region 130 of the transparent substrate 102. FIG. 2C shows an inspection signal pattern 170 generated by the first mask database 160.
FIG. 3A shows a second mask database 180 inside a built-in database for designing and fabricating the chrome-less mask 100. The second mask database 180 includes a light-blocking region corresponding to a light-blocking pattern 164 for removing the chromium layer 104 over the phase shift region 120. Using the second mask database 180, a mask with a cross-sectional structure as shown in FIG. 3B is produced. FIG. 3C shows an inspection signal pattern 190 generated by the second mask database 180.
A comparison of the signals 150, 170, 190 in FIGS. 1C, 2C and 3C shows that the actual inspection signal pattern 150 is different from inspection signal patterns 170, 190 generated by other designing patterns. In other words, signals generated by the designing pattern of databases do not reflect the signals produced by an actual chrome-less mask. That means, the die-to-database inspection method cannot be applied to detect defects in a chrome-less mask.